The present invention relates to a primer for mineral substrates, especially for those substrates that react in a strongly alkaline manner, for example owing to their content of hydraulic binders which have not yet been fully reacted, such as cement, lime and the like.
The substrates mentioned generally have porous structures as a result of which, owing to the absorbent properties of the substrate, it is difficult, if not impossible to achieve uniform application and satisfactory coating when applying coatings for improving durability, for protecting the surface and/or for embellishment. Furthermore, in many cases, after the penetration of moisture into unavoidable cracks, such coatings lift from the substrate.
To avoid those difficulties, primers of various compositions, which are intended to isolate the coating from the actual substrate, have already been used. They are applied, for example, when the basic material of the coating is not compatible with the substrate. In this context, reference is made only to the known incompatibility of poly(organosiloxanes) with substrates that react in a strongly alkaline manner and to the low permanence of polyvinyl esters in alkaline media.
Solutions of organic polymers in organic solvents have often been used as primers (see U.S. Pat. No. 3,716,399). The desired isolating action mentioned above has not, however, been found in that case since those solutions were absorbed, for the most part completely, by the substrates. Furthermore, the use of such solutions can scarcely be justified any longer for reasons of work and environmental hygiene and also because of the risk of fire.
It has also already been proposed to use aqueous polymeric dispersions having polymer particles of approximately from 0.02 to 0.1 .mu.m (see , for example, DE-A-25 31 895 or the corresponding U.S. Pat. No. 4,089,999), which have a satisfactory penetration capacity but which tend to form a gloss where the substrate is not porous. Furthermore, the desired reduction in water adsorption is often not adequate, especially in the case of crack formation, and it is also often difficult to achieve the desired stabilization of the substrate.